1. Field of the Invention
This invention relates to rechargeable battery packs having a plurality of series-connected cells, and more particularly to battery packs adapted for implantation in a human body and an apparatus and method for charging such battery packs.
2. Background Information
Rechargeable battery packs are a packaged assembly of a plurality of interconnected cells or “batteries” that are typically joined together to generate a desired overall voltage potential and current for powering a desired device or system. The term “battery” is sometimes used to describe a multiple-cell arrangement, and may often be used interchangeably. However, an extremely wide variety of modern electronic devices are powered by “batteries” that are configured as multiple-cell packs with particular voltage and current-handling capabilities tailored to the respective device. A large range of sizes and types of battery packs are produced to power these devices. Batteries typically employ a combination of electrolytic members and or electrolytes that undergo chemical/ionic transfer reactions to produce electric current. During current discharge, the components of rechargeable battery packs undergo a series of complex chemical reactions in a particular “direction,” with concomitant change in the makeup of the electrolytes. Conversely, recharging, which uses an external current source to force current back into the battery pack, causes the direction of the internal electrochemical reaction to reverse, thereby returning the electrolytes to their original charged state.
Types of rechargeable battery packs in common use today include lead-acid, nickel-cadmium (NiCd), nickel-metal hydride (NiMH)—and the increasingly more-common lithium ion type. Each of these battery types is generally characterized by a multiplicity of individual cells, each containing the basic electrode/electrolyte combination that produces a discrete voltage potential thereacross. The opposing electrodes of a given number of cells are connected together in series so that the additive effects of the individual cell potentials are combined to produce the overall desired voltage potential for the battery pack.
One disadvantage associated with multiple-cell battery packs, including lithium ion type, is that the cells are not exactly identical in performance and charging characteristics. For example, within a given charge duration, one cell may reach a higher potential than an adjacent cell. A common charging technique is to regulate the application of charge current so that the charging current is cut off when the highest cell reaches its potential. However, this form of regulation may leave the other cells in the battery pack undercharged. This reduces overall battery capacity and may shorten pack life.
Various techniques have been employed to attempt to balance the potential in each cell. This may involve the use of a somewhat inaccurate mechanism for roughly balancing cell-to-cell potential between respective cells—or where a more accurate balance is desired, the use of fairly complex and expensive monitoring circuit, that increases the cost of the battery, may be required.
Multiple-cell lithium ion battery packs have become a common standard power source for portable electronics, and are desirable for their low-weight, small size, high power output and long life. Many common types are commercially available in the 9 to 14-volt output range, having up to four series cells each delivering approximately 4.2 volts. The advantageous characteristics of lithium ion type batteries render them particularly suitable for critical medical devices such as cardiac implants and monitors. However, where such battery packs are to be employed in the extremely demanding medical environment, reliability, quick charge capability and long life take on even greater importance. This is especially true where batteries may require high output (24–30 volts DC) and are directly implanted into the human body to service an implantable, life-saving device such as a pacemaker, cardiac assist device or artificial heart or other heart-treatment device.
Accordingly, it is an object of this invention to provide an apparatus and method for charging a multiple-cell battery pack that ensures all cells are charged to peak potential, thereby maximizing power output and life. The apparatus and method should enable a large number of high-output cells to be efficiently charged, with no undesirable delay in the charging process. The battery pack should enhance overall reliability in critical applications such as in conjunction with life-saving and/or implantable devices.